text:
How is a Flying-V more efficient than current fuselage-over-wing designs?

It's claimed the plane will use 20% less fuel than the Airbus A350-900 while carrying a similar number of passengers -- the Flying-V will seat 314, while the Airbus A350 seats between 300 and 350. The design also mirrors the A350's 65-meter (213 feet) wingspan, enabling it to use existing airport infrastructure.

Source: CNN
All the efficiencies in recent years have come from better & better engines, huge high-bypass turbofans.  This fuselage looks like it'd be a lot draggier than existing fuselage/wing structures since there is so much more frontal area.
If you have a window seat, that window cover better be open if I'm in that row!

Cargo doesn't have many of the issues passengers do, and we still don't see BWB specific for that.

Flying-V is an investment scam.

As I understand it, the flying wing lends itself to more efficient loading and cargo volume. A normal airplane can only carry fuel in the wings and volume maxes out if you are carrying potatoes chips. A blended wing offers a choice of cargo volume or weight!

There are fuselage tanks also.

I always ask myself the same question when I read such claims. Only that I am concerned about the huge wetted surface - frontal area is less of a problem here.
When I get hold of such studies, they invariably use projected future technology and compare that with current standards. As you correctly observe, most progress in fuel consumption over the last half century has been due to better engines with higher bypass ratios. Improved aerodynamics contributed maybe 30%, helped by higher aspect ratios and better surface quality made possible by using composite structures.
Another trick: BWBs (Blended Wing Bodies) with their wide fuselage are assumed to need less time for embarking and disembarking of the self-loading cargo. That will make them more productive but the complication of emergency egress from this wide fuselage (remember - all on only one side, within less than 90 seconds!) are waved aside. Also, structural penalties for a flat pressure vessel are assumed to be ridiculously low, so the BWB is even claimed to be more efficient structurally. Which it is, if you look at the spar weight alone. But skin surface area has weight, too -  but not in those studies.
What also helps is to fly more slowly. That improves engine efficiency especially with high bypass ratio engines and allows thicker airfoils which certainly are needed in order to spread passengers over more than half of the wingspan. What is saved on the ground is then used up in the air, and the result is indeed a more efficient operation. If we were not bound by that magic number of Mach 0.85, new designs could indeed be more efficient still.
Remember that the A380 also was claimed to be 20% more fuel efficient? After being overweight (no wonder if scaling laws are observed, something the base of this claim conveniently forgot) what is left of that claim today? In reality, the competition uses the same technological progress and the comparison has to be made on a level playing field, with real data. Then those claims quickly collapse.

@ymb1: Yes, but the real competition is the 777-300 ER. The 747-400 flew 20 years before the A380 - of course those 20 years brought an improvement by 10-15% - something that the 747-8 also achieved.

I know I can always count on @PeterKämpf for a thorough treatment of issues like this!

@ymb1: The 20% were the demand of Airbus management and of course quickly picked up by marketing early in the development of the A380. You are right that later, with better knowledge, claims of only up to 15% were made.


text:
Why don't horizontal stabilizers have winglets?
Winglets are used to reduce induced drag on the main wings of an aircraft as per explanations on wikipedia. Since they are very effective I was wondering why they are not installed also on horizontal stabilizers. For sure there must be some sort of induced drag being generated on horizontal stabilizers too as they are cutting through the air just as the main wings do. So why aren't there any sort of "minified" winglets available as an aftermarket installation (or, at least, I haven't ever seen some myself)?

Induced drag is caused by the pressure difference over and under the wing meeting at the tip generating a vortex, not by "cutting through the air" (that's straight up drag, not the 'induced' kind)

Or in other words, induced drag is induced by producing lift.

Horizontal stabilizers don't generate as much pressure difference as the wings. Generally the stab deflections are very small in flight, and there's so much other drag during landing -- when the stab gets the most use -- that wing vortices from the tail are probably the least of your problems. Stabilizer winglets on their own, when there's no pressure difference, would simply result in drag, whereas the main wing winglets are constantly effective while flying.
Furthermore, the weight and trouble of adding little winglets to the horizontal stabilizers with larger actuators and hinges, for example, would probably outweigh the very slight aerodynamic benefit. I could also imagine that since the airflow over latter part of the wing is complex and varied, find a good design solution would be difficult.
As @Federico points out, the DA42 has them, but that's a composite airframe rather than the metal construction you see in most Cessnas, for instance, where you have to be more conservative in construction.

Short answer: In all cases these are not winglets, but fins.
Winglets are worse than an equal span extension and are only used if span should not increase:

for limiting the wing's root bending moment, or
for size limitations

Putting them on the tail surface would not help to limit the wingspan: The horizontal tail can easily grow and still be much smaller than the wing. There is no single case where winglets on a horizontal tail would have made sense.
What you see and might interpret as winglets are actually additional fins. They stay in the general area of the vertical tail, so they do not increase the sideslip-induced rolling moment, and they do not increase aircraft height. But they help to improve lateral stability, which had been worsened by a configuration change. Example pictured below: The Beech 1900, a derivative of the Beech Super King Air. The fins below the fuselage could not grow any bigger without risking ground contact during rotation, and the small stability deficit did not justify the development of a new tail section. So the fins were added where they would cause the least amount of trouble. Obviously, aesthetics were not considered.

Note the small planes at the bottom of the vertical tail: The same trick was used to avoid increasing the size of the horizontal tail. It could be argued that the horizontal tail of the Beech 1900 is a double decker.
And no, those horizontal fins are not "fuselage winglets".
So-called "winglets" on the tail of the DA42
That is also the reason why the "winglets" are so much bigger on the MPP version of the DA42: They offset the destabilizing effect of the camera pod. The canted tips of the original DA42 are working like an anhedral and create a compensating positive rolling moment for the vertical tail's negative sideslip-induced rolling moment. Ideally, the whole horizontal tail would have anhedral, but it was preferred to keep the elevator hinge line straight.

The 1900D's tail region looks like something designed in Kerbal Space Program.

So why aren't there any sort of "minified" winglets available as an aftermarket installation?

Actually the DA42 has them

and in the MPP version they are even more pronounced

i'm not sure these are the pressure difference purpose like classic winglets. IMHO just from appearance it looks like the DA42 has a very short rudder, so these might be for stability purposes :)

@Manfred actually pilots I have spoken to report that these worsen the stability, not improve it.

Look at the camera platform of the DA42 MPP: This decreases lateral stability, and the so-called "winglets" are simply additional fins to bring lateral stability back to normal. These are not winglets at all, but fins

@PeterKämpf call them the way you want, the pilots report a loss, not a gain, of stability with them installed (and no camera platform)

@Federico: What stability? Lateral or longitudinal? Dynamic or static? Not even some test pilots get that sorted out correctly.

@rbp: The strakes are for stability at high sideslip angles. At small sideslip the vertical tail will do fine all by itself, but it will run into a stall around 15° of beta, and then the strakes help out. But this is all besides the point. The stability of the whole configuration is finely balanced, and then that camera pod is added in front. So now something must be added in the back to regain lateral stability, and that is what the fins are for. They were never intended as winglets, even if they might help the horizontal tail a bit.

The induced drag from wingtip vortexes is mostly on lift generating surfaces, horizontal stabilizers don't generate that much lift.
That said there is a twintail design with double vertical stabilizer at the ends of the horizontal one; but that is not for reducing vortexes but for improving rudder response.

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text:
What aircraft can make these sharp 90-degree turns?
I took this photo when I saw a plane make a crazy 90 degree turn. What kind of aircraft could do this? I have a plane finder app for commercial aircraft and this plane did not show, which is the first time that's happened.

Any plane can turn 90°. What you are hinting at is the aircraft with the ability to turn at the highest curvature. http://en.wikipedia.org/wiki/Curvature

Do you have any idea how long it took from starting to ending the course change?

Although the contrail looks quite unbroken, it is very possible that strong winds aloft have pushed the lower section (at about 270 degrees) into a more right angled appearance.  Contrails often do not show the path an aircraft actually took.  What makes you think that what you saw is "crazy"?

@Simon: This could even be two independent contrails, stopping rsp. beginning at the boundary of two air masses. Only the air mass on the left side would make contrails stable.

The perspective can be misleading. The turn was not more than perhaps 50° and at least 50 km away, so it did not even have to be particularly sharp.

The course change was quick. It was one plane. The idea that , maybe the wind blew the contrail to look as though it took a much tighter turn, seems possible. I was watching the plane, I looked at my phone to see where it was going on a 'plane finder' mobile app. In the next few seconds, it had turned. It flew over me, as I was on the ocean, I could see 4 plumes of exhaust coming out. The mountain it flew behind is 1700 feet , so it couldn't be much higher than that

Not on PlaneFinder suggests military. I don't think it's low, and your perception of it passing behind a mountain is just perspective since contrails don't usually form below about 20K feet. In your photo, it appears much higher to me. 4 contrails would then suggest a military heavy, e.g. KC135. Civil jets tend to make shallow turns, typically 30 degrees of bank or less, mainly for passenger comfort, but a KC135 is capable of much higher rates of roll, and therefore much tighter turn radii. It may also have been a training manoeuvre.

Thanks everyone. Your responses are amazing. I appreciate you all giving this some thought. It's very interesting to hear other thoughts about this

"It was aliens".

It looks like a 90 degree turn, but it isn't. You're not looking vertically up at it; you're looking at a fairly shallow inclination, maybe only 20 degrees elevation at the corner of the turn. It means that the actual course change is probably only about 20 or 30 degrees, and just looks like more because of the optical effect of foreshortening. As for the aircraft, pretty much anything capable of flight at high altitude - probably 30,000 to 40,000 feet - any jet airliner or many business jets.

We looked at how tight a turn a Boeing 747 can make in this question. The answer gave a radius of just 6.11 km.
Your contrail photo clearly shows that the airplane has either one or two fuselage-mounted engines. The 747 was assumed to have a limit of 1.5g for the 6 km turn, but if the aircraft in your photo has a higher buffet and airframe limit, much tighter turns would be possible.
The turn radius $R$ can be determined when airspeed $v$ and bank angle $\Phi$ rsp. load factor $n_z$ are known: $$R = \frac{v^2}{g\cdot tan\Phi} = \frac{v^2}{g\cdot \sqrt{n_z^2-1}}$$
If your airplane was traveling at Mach 0.7 in FL300, its speed would have been 212 m/s. Let's further assume it could pull 3g at this speed and altitude, and your turn radius becomes 1.624 km or just about one mile.
It is very hard to judge the radius of the contrail's turn section, but it is quite easy to list the characteristics which help to make the radius small:

Low wing loading
enough thrust to sustain high load factors
enough strength to allow for high load factors
enough lift capability to allow for high load factors

One could add the capacity to fly high enough to be able to form contrails, but this is already implicitly contained in the other conditions. We need something with big wings and a powerful engine - a lightly loaded fighter aircraft in other words.

@jjack: If the engines are under the wings, you will see two or four distinct contrails, especially when the contrail is fresh.

As we do not know which airplane were you observing and neither the airspace it was moving in, one possibility, aside for military aircrafts, it could be that it was an acrobatic one, maybe training on specific maneuvers.

Such a turn from a long distance could look like it is seen in the photo you posted.

Wouldn't it be strange that it flew perfectly straight for most of the time? With smoke on?

@PeterKämpf 1. it can be contrail, why assume it is smoke?
2. if it is training, you prepare, concentrate and do the maneuver in the cleanest possible way, thus flying straight most of the time

The first part of the contrail disappeared seconds later, as my next photo didn't show its initial flight direction. I was on a board, on the ocean when I took the photos. The plane made the turn & flew over me & behind a mountain , which is 1700 feet. So it wouldn't have been flying much higher than that.

As Federico said, there is definitely a possibility of an acrobatic or stunt aeroplane, and the first thing that came to my mind when I saw this picture was an Extra 300S. The contrail looks to me like more of a smoke trail (Extra 300 series planes are usually equipped with smoke generators), as towards the beginning of the line the trail seems to be 'bunched up', or wavering. Also, a couple hundred feet (at best guess) from the turn, the line shifts, possibly as a result of wind shear and/or turbulence.

Unlikely to be a sport/aerobatic aircraft - it's a high altitude contrail. Sport/aerobatic aircraft are unlikely to have a ceiling that high, and would have no reason to have smoke turned on.

"When you see hoofprints, look for horses, not zebras."

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Does European airspace use the metric system?
I figured this was probably a no brainer, since Europe uses the metric system, I assume the airspace does...  But then I remembered that aviation likes international standards, and I know the US uses Imperial so, does Europe as well?
And, yes or no, why do they take that choice?

European Civil Aviation Authorities use the imperial system to depict altitudes and airspace restrictions (feet), speeds (knots) and distances (nautical miles).
Some VFR aircraft have speed indicators in km/h, gliders can have altitude and variometer in meters and meters/second and apart from a special ICAO VFR chart being available in metric in Germany, the AIP itself and all other charts always use imperial units.
Related question: What is the measurement system used in the aviation industry?

Europe uses imperial for distance (nm), speed (knots) and altitude (feet). Russia and China are the only major countries that use meters for altitude - and it causes all sorts of confusion.
Some aircraft (more in europe) use kilometers and statute miles/hour on the ASI - but the same is true in the US.

The nautical mile is now (since 1954) defined in terms of meters...

@copper.hat As are all US customary units of measure that I'm aware of; the yard was first defined in meters in 1893, and was revised in 1959.  Other units of measure are similarly defined in terms of SI units, or in terms of units defined in terms of SI units.

@cpast: I generally have a preference for metric (consequence of a childhood spend doing multi-base arithmetic with acres, roods, furlongs, pounds, schillings & pence), but prefer the nautical mile because of the relationship with the Earth's circumference.

Russia has already switched to feet-based flight levels, though they still use metres below transition altitude.

There are aircraft that use nautical miles for distance and statute miles for speed?

Distance isn't really used for a particular aircraft - but I can't think of DME that will display anything else than NM, and they are certainly installed in aircraft with statute miles on the airspeed indicator.

As others have mentioned, the basic units to define the airspace are feet and nautical miles.
However, some other values are also used when navigating the airspace, and a few are from the SI system. For instance:

pressure settings: hPa (aka millibars, mbar)
runway lengths: meters
visibility: meters
temperature: centigrades

Regulations (VFR):

minimum horizontal distance from obstacles: meters

To see the SI units for pressure, visibility and temperature in a real-life setting, just have a look at a TAF/METAR for any European airport.
The mixture of units can seem strange, but there is also a method to the madness. You know that whenever you hear a measurement in feet, it is about a vertical distance, and  anything expressed in meters (or kilometers) will be horizontal. A small contribution to the clarity of communications.

I've most commonly seen runway lengths in feet, even on airport diagrams. It's true that metric value is often given as well, but doesn't seem to be the primary thing.

@yo' Just checked the official information for Heathrow, EGLL, Charles de Gaulle, LFPG and Schiphol, EHAM. That's three of Europe's largest airports, and they all uses meters for RWY length (and width). Are you sure you didn't see a US "translation" (Jeppesen or similar)?

Ah yeah I used Jeppesen, right.

These are the critical differences.  European ATC will variously give altimeter settings in hPa but pilots can request inHg, afaik (if they don't have dual-legend altimeters, for example)


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Why are gliders oftentimes not equipped with a transponder?
Oftentimes gliders are not equipped with transponders (at least in Europe). Even if they are, they do not necessarily operate it at all times. This is allowed by SERA.13001:

(a) When an aircraft carries a serviceable SSR transponder, the pilot shall operate the transponder at all times during flight [...]
(c) Except for flight in airspace designated by the competent authority for mandatory operation of transponder, aircraft without sufficient electrical power supply are exempted from the requirement to operate the transponder at all times.

Modern gliders give a bad target for primary surveillance radars, because they are small and contain little reflective material. Thus gliders maybe unknown to air traffic controllers and flight information services. A transponder would give ATC secondary surveillance radar data which could help to avoid mid air collisions between gliders and other planes, especially airliners which may be too fast for practical "see and avoid" (as e.g. demonstrated in the event of an airprox between a glider and an A321 near Hamburg, Germany).
I could not find definite sources about the power consumption of transponders, however this question implies an average consumption somewhere between 2W and 6W. Considering that USB power banks with a capacity of 100Wh are small, light, cheap and could easily provide the energy needed for >16 hours (a time I expect should cover >99.99% of all glider flights) it seems that technically the power supply should not be the reason for not operating a transponder.
Another reason may simply be money. A quick google search implies about 2000 € for equipping a glider with a transponder which seems quite a lot considering that this is quite a simple device (e.g. compared to a smartphone). Nevertheless it is quite an easy measure to increase flight safety.

So this received 2 downvotes. I have no idea why. It would be nice if the next one leaves a comment about what is wrong.

I see nothing wrong with this question. While not required, it would be very helpful to leave constructive criticism in the comments when downvoting.

"without sufficient electrical power supply" seems to be your answer... No engines = no generators (I'm sure glider pilots don't want ram air turbines either). What if the pilot forgets to charge the battery at home?

And don't forget those batteries and transponders, especially for duration flights, mean quite a bit of extra mass which reduces range and performance. And that's the last thing you want as a competition glider pilot especially.

A useful question, clear and well researched, offering apparently valid solutions and asking why these solutions are not used to prevent a risk of collision... one upvote, 3 downvotes.

@user253751 pilots could forget many things required for a safe flight. Training and carefully following check-lists prevents that. New item #1 on the checklist: "Charge the battery today for tomorrow's flight".

Perhaps because collisions of airliners and gliders are vanishingly rare.  This SE question found exactly one instance, in which both planes landed safely: https://aviation.stackexchange.com/questions/80936/has-any-airliner-ever-collided-with-a-glider

The downvotes might be from glider pilots who don't like the idea of another expensive regulatory requirement imposed on their hobby, rather than because the question isn't clear or well-researched.

Here's the UK CAA page about the Electronic Conspicuity rebate scheme and the devices that are included:  https://www.caa.co.uk/General-aviation/Aircraft-ownership-and-maintenance/Electronic-Conspicuity-devices/   (this time with a link to the page!)

Gliders in Europe are not usually equipped with transponders because most of them have FLARM.

As of November 2017, over 35,000 manned aircraft already have a FLARM
system installed, including both the legacy Classic FLARM models and
the PowerFLARM models. Most FLARM installations are in Europe. Over
50% of all GA aircraft in Europe already have FLARM and thousands of
installations are currently being made each year.
In Switzerland, almost all powered airplanes and helicopters have
FLARM. In the rest of Europe, most powered airplanes and helicopters
have FLARM.
In Switzerland, Germany and France, all gliders have FLARM. In rest of
Europe, almost all gliders have FLARM.
In the UK, over 7,000 aircraft already have FLARM, of which over 50%
is powered airplanes and helicopter.
Additionally, FLARM is in use in over 20,000 drones/UAS

FLARM is lower cost and lower power consumption than most ADS-B or Mode-S, and better fits the way that gliders and light GA use the airspace.  Gliders tend to fly in Class-G airspace and/or close to their base airfield.  The traffic they usually encounter is mostly other gliders or powered GA flying VFR.  As jamesqf pointed out, encounters between gliders and airliners are extremely rare.  TCAS is designed for IFR traffic and its purpose is to maintain IFR minimums, and while gliders are allowed to fly IFR, I think it's usually well out of the way of airliners.

Exactly. This should be the accepted answer.

The problem with FLARM is that it does not help to avoid collisions between light aircraft and airliners (in airspace class-E), because airliners are not FLARM equipped (something that might change in the future). While encounters between airliners and gliders are rare, this is still a huge topic in commercial aviation, as a single accident will have a huge effect on the airline's safety record. So basically I interpret your answer as "Within the glider community the risk of conflict between gliders and airliners plays a hardly any role and within the community FLARM is used"

TCAS will generate a resolution advisory with VFR traffic, so no, it is not just for IFR traffic. TCAS is not programmed to maintain separation minima, but to prevent collisions.

Since low power ADS-B out is already available for medium to large drones, I can envision that gliders might start to adopt it. Likely we'll see "dual" systems for both ADS-B and FLARM in gliders in the near future, and I would predict FLARM being slowly phased out.

This is anecdotal, but as a glider owner I can tell you why my glider doesn't have a transponder:

Monetary cost: A transponder is a substantial investment which requires recurring inspection. Avionics shops tend not to set up shop out in the boondocks where gliders fly.
Energy: a transponder consumes a fair amount of energy and the battery in my plane is not easy to access to remove. And anyway there's no convenient way to charge it when it's sitting in a trailer several hundred meters away from the nearest electric socket. FWIW, 2-6W is a huge power load for a glider, the electric variometer I use can run a year off a 9V battery.
Weight: we go to great lengths to make the glider light. Adding back in a radio and suitable battery would work counter to that.
Regulatory burden: If I had an ADSB transponder, then the plane would not be legal to fly with the transponder off. This means that if the battery is dead, the plane is grounded.
Value: Ignoring the specific problem of gliders flying in a gaggle-- a situation where a transponder doesn't help-- mid-air collisions are extremely rare. And when we're flying in the mountains, ATC couldn't receive the signal anyway. A FLARM, as Dave Gremlin describes in his answer, system is far more practical.

Of course, change the equation variables and my response changes. I don't speak for other owners, but I suspect they have similar priorities.

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text:
Why do the DC-9’s three-phase electrical systems use separate, unconnected breakers for each phase?
Aircraft electrical systems use two types of electricity:

Direct current (DC),1 which always travels in the same direction (can be supplied directly from the aircraft’s battery, if necessary, but usually obtained from the AC electrical system through a rectifier).
Alternating current (AC), which reverses direction many times every second (usually supplied by the engine-driven generators, or, sometimes, the APU-driven generator, but, in an emergency, can also be obtained from a ram-air-turbine-driven generator or by passing the battery’s DC output through an inverter).

Most of a typical aircraft’s AC electrical system uses single-phase AC, with just one set of wires; however, a few electrical components (primarily heavy motors) require three-phase AC, with three sets of wires.  Like any other electrical circuit, each of those three phases is protected by a circuit breaker.
Accepted engineering practice for three-phase circuit breakers calls for mechanically linking together the breaker mechanisms for each of the three phases, so that a fault with any one of the phases trips the other two offline as well (preventing electricity from the non-faulted phases from going places it shouldn’t).  Indeed, googling “3 phase aircraft circuit breaker” or the like turns up images and descriptions of breakers with all three phases sharing a single mechanical toggle.
This standard practice was apparently not followed, however, for the three-phase breakers on the DC-9 (or, at least, one particular DC-9; all emphasis and [comments] are mine):

At 1851:14 eastern daylight time [...] the three circuit breakers associated with the aft lavatory’s flush motor and located on a panel on the cockpit wall behind the captain’s seat, tripped in rapid succession. (The motor is a three-phase alternating current (a.c.) motor; each phase incorporates a circuit breaker for protective purposes.) After identifying the circuit breakers, the captain immediately made one attempt to reset them; the circuit breakers would not reset. The captain assumed that the flush motor had probably seized and took no further action at this time. About 1859:58, the captain again tried unsuccessfully to reset the three circuit breakers. According to the cockpit voice recorder (CVR), he told the first officer that the circuit breaker(s), “Pops as I push it.”  [Page 7, numbered as 2.]
[...]
The 5-ampere phase-A, phase-B, and phase-C flush motor circuit breakers were removed from the cockpit and were X-rayed, revealing no internal damage. During a functional test, all three circuit breakers tripped when the electrical load exceeded the 5-ampere rating. The circuit breakers were connected to a power source and load and subjected to a 100-percent 10-ampere current overload. Only the phase-A circuit breaker exceeded the time limit designated in the specifications, before it tripped. The Safety Board could not determine the reason for the failure of the phase-A circuit breaker to meet its specifications; however, all three circuit breakers showed evidence of damage due to an external heat source.  [Page 42/37.  Had the breakers for the three phases been mechanically interconnected, as per standard engineering practice, the phase-A breaker would have been forced to trip at the same time as the other two.]
[...]
Individual units are protected by individual trip-free circuit breakers. These, when tripped, cannot be electrically reset until cooled. Those units requiring three-phase supply will have individual breakers in each phase. [Page 49/44.]
[...]
Another possible source of ignition near the area where the fire was discovered was the flush motor wiring harness. The tripping of the three circuit breakers accompanied by the arcing sounds recorded by the CVR occurred at 1851:14. The three circuit breakers tripped almost simultaneously indicating that the circuitry of all three phases shorted at the same time. The only evidence of wiring damage was found where the flush motor wiring harness passed through the lightening hole in the partition between the amenities section and the toilet section of the lavatory. The damage noted in the wiring harness at this location could only have been the result of fire and heat, and the Safety Board concludes that the damage to the wiring which caused the three flush motor circuit breakers to trip was caused by heat and fire.  [Page 60/55.  Had the three breakers been mechanically interconnected, they would all have tripped simultaneously as soon as the first of the three phases shorted out, regardless of whether or not the other two had faulted.]
[...]
At 1851:27, the captain tried unsuccessfully to reset the circuit breakers. At 1859:58, he again tried unsuccessfully to reset the three circuit breakers. Air Canada flightcrews are taught to make one attempt to reset a tripped circuit breaker. They are taught that it may be necessary to allow a 3-minute cooling time before a circuit breaker will accept a reset and that circuit breakers protecting a single phase are trip free; therefore, a circuit cannot be completed by holding in an unlatched single-phase circuit breaker. Most important, they are taught that a tripped circuit breaker denotes that the circuit protected by the circuit breaker is no longer powered. The flightcrew and other Air Canada flight personnel stated that circuit breaker trips during flight are not an uncommon occurrence, and the procedure contained in Air Canada’s DC-9/AOM allows personnel to cope adequately with such occurrences.
In this case, the captain attempted to reset each of the tripped circuit breakers twice; the first attempt occurred almost immediately after they had tripped and was unsuccessful. He testified that he “thought at the time that the unit (flush motor) might be overheated so I just continued the routine of the flight... and after a certain time had passed... I attempted to reset the circuit breakers again to make sure. ... The circuit breakers would not move.” Although the captain was unable to detect any movement of the circuit breakers, the CVR showed that arcing sounds, which were not audible to either the captain or first officer, accompanied each attempt to reset each circuit breaker indicating that the circuit breaker had moved and momentary electrical contact had been made. However, once the contact was made, the protective circuitry caused the breaker to trip again. Since the fire was already well established, the attempts to reset the circuit breakers had no effect on the sequence of events. About 1902:40, 11 minutes 26 seconds after the initial trip of the three flush motor circuit breakers, the No. 2 flight attendant informed the flightcrew that there was a “fire.”  [Pages 64-65/59-60.]

Why aren’t the DC-9’s three-phase electrical systems protected with proper three-phase breakers, instead of individual separate single-phase breakers for each phase?

1: No relation to the DC in the names of Douglas airliners, to the comic-book company, or to the seat of the U.S. government.

A three-phase electric motor can in some cases continue to run on only two phases but at reduced power, depending on the nature of the load it is driving.  Having separate breakers for each phase would allow this.


text:
Why are Category II ILS approaches prohibited in an E-Jet with inoperative pitch trim?
Part of the Pitch Trim Runaway checklist for the E-Jet family states that Category II ILS approaches are prohibited if neither pitch trim channel is available:

If both pitch trim channels are failed:
Cat II Approaches are prohibited.
[E170/175 QRH, page 8-32, quoted in page 2 of NTSB ASR-20-01]

Why are ILS-II approaches (but not ILS-I or ILS-III approaches, or any non-ILS approaches) prohibited in an E-Jet with inoperative pitch trim, even if adequate elevator control is available to do so safely?

Are you sure that Cat III approaches are allowed in that condition? Typically, Cat III requires basically everything to be operative, unless there is some pretty unusual redundancy in the aircraft.  Wouldn't be surprised if one channel out removes Cat III, and then losing the 2nd channel takes out Cat II. Just a guess, but the list of things required for Cat II that aren't needed for Cat III, while not empty, isn't very long in most cases.

@RalphJ: In that case, one would expect to see "Cat III approaches are prohibited" or something similar at an earlier point in the Pitch Trim Runaway checklist, which isn't the case.

If it's not certified for CatIII it won't make any mention in the first place.  If the airplane didn't have head-up guidance, it can't be cert for CatIII.

@JohnK: HUD or (more commonly) triple-string autoland.

I see cockpit photos of E-175's that have HUD's in them, but the photos I see of Republic E-175 cockpits don't show a HUD.  Unless you know that the Republic aircraft in the report was both equipped and approved for Cat III ops, then the inference behind the question seems a little baseless. If the crew can't fly any form of Cat III approach anyway, you wouldn't expect the checklist to address it.

The Regionals like the CRJs and Ejets are cert for CatIII without needing autoland.  They just need HUGs.  There is no autoland system available for those airplanes.

Looking at the report, the airplane does not appear to be certified for CatIII in the first place (it would have to have Head Up Guidance and not that many do - the CRJs are the same) so the QRH doesn't need to mention an approach category that the airplane can't do anyway.
As far as CatII goes, the approach with no pitch trim is going to have to be flown by hand, and probably with the existing trim speed different from Vref, unless the trim failure happened while on final while already trimmed to Vref.  So depending on how far away your current, fixed-due-to-failure trim speed is from Vref, you are going to have the hand fly an ILS while holding 5, 10, 15, 20, 30 pounds of force on the column to maintain glide slope at Vref.
This is quite a challenge because the slightest relaxing of concentration will cause you to relax your push/pull a bit and make you drift high or low.  It's enough of an ordeal to do this and stay on glide slope right down to 200 ft.  To hold it down to 100ft, the normal CatII decision height, is pushing it, so the approach is prohibited.

So the problem isn't with it being an ILS approach, but, rather, with it being an approach to very low minima in an out-of-trim aircraft?

Pretty much.  Keep in mind that  Cat II approaches generally require all of certain key systems working so there will be various types of system failures where you have to stick to Cat I minima.  If you peruse that QRH more you should find a number of conditions where that prohibition statement for Cat II appears.


text:
What is the ratio of standby crews to flying crews?
Section 4.O of the FedEx Collective Bargaining Agreement discusses how pilots are to be compensated for being on standby. From what I can tell they basically get paid a "trip guarantee" that is close to the same as had they flown the leg, even though they may not get any flight time (right?).
That made me wonder: How many standby crews does an airline schedule for a given number of flights? Is it 1:10? 1:50? Does it differ between passenger airlines and cargo airlines?

Related : https://aviation.stackexchange.com/a/25631/679

related: What is a typical ratio of airline pilots to aircraft?


text:
What do the different dates in a TCDS mean?
I have been trying to interpret Type Certificate Data Sheets and am coming up short on how to understand some of the dates listed. For example, here is the TCDS for the new Citation Longitude:

How is one to interpret these dates? Did the type certificate get issued in 2020 or 2019? The Textron Aviation website has a news release in September that claims certification occurred in 2019; so is the 2020 date specific to the TCDS being released, even though FAA approval occurred in 2019?
Also, how does this related to the Revision numbers? Does the 2020 date refer to Revision 4 and the 2019 date refer to a different revision? I can't seem to find the earlier revisions on the FAA website either, so I am not sure whether they even exist as official/released FAA documents.

The Type Certificate document is just a kind of diploma so to speak, or, as it says in its name, a Certificate.  The real meat is in the TCDS, which describes the technical details, limitations etc., of the aircraft that are the basis of its Type Certificate and which set out the parameters that must be met to be in compliance.
When these details change or need to be corrected, they don't reissue the Type Certificate, they just revise and reissue the data sheet that is linked to the TC by its number, T00015W1.
In your example, Feb 5 is the date of issue of the revised Data Sheet at Rev 4, so there would be three other revisions, plus the original issue (which should have the same date as the TC itself), prior (Page 6 is still at Rev 3, and Rev 4 is Page 1-5, and revision bars show what was changed in Rev 4 on those pages).  That's why it's in the document identification block at the top right.  It's the issue date of that Data Sheet revision.
The TC approval date is just a reference to the original TC issue in Sept 2019, which doesn't normally change, being the fancy sheet of paper you frame and stick on the wall when you're the engineering VP who led the program.


text:
How to calculate the radius of a turn as a function of speed, bank, wing-loading, and Cl?
Although the question of how to calculate the radius of a turn of an aircraft has been answered before using only the speed and the bank angle, the reality is that 2 aircraft with the same speed and bank angle will have a different radius of turn if their wing loading is different.
Furthermore, two aircraft with the same speed, bank angle, and wing loading will not necessarily have the same radius of turn. The lift coefficient of the wing must be taken into account.
Since the answer posted before is not complete, can somebody provide an equation that takes into consideration the above factors (and any others that should be contemplated)?

Why do you think wing loading affects turn radius? It doesn't, but if we can understand your thought process better, maybe we can provide a more useful answer that addresses the misconception rather than rehashing the existing Q&A's.

Please, see my comment for Answer 1. Thank you

The formula for turn radius is Velocity squared divided by radial G. And since Radial G, (assuming a level turn) is absolutely dependent on bank angle, the turn radius IS absolutely dependent on Velocity and bank angle. No other factors are necessary.
You are incorrect that two aircraft with different wing loading will have different turn radii at the same airspeed and bank angle. Anyone who has flown formation, even if they are not familiar with the physics/mathematics involved, knows this from experience.

From the book Fighter Aircraft Performance of WW2 A Comparative Study (Pilawskii, Erik), you can find: "Aerodynamically speaking, a design's wing loading plus its maximum lift coefficient Cl will determine the ability to turn in a small radius in the shortest possible time". Comparison testing of different fighters showed that they did not have the same turning radius when starting from the same speed and using the same bank angle. That is the reason of my question.

Wing loading determines the MAXIMUM turn performance, or, in other words, how tight an aircraft can turn before it stalls. . In other words, a an aircraft with high wing loading, as it turns tighter and tighter, will reach stall at at lower bank angle and turn rate than an aircraft with lower wing loading. But as they both increase bank angle and turn rate, flying together at the same airspeed, they will have identical turn rates, bank angles  and radial g.

I also found this study that includes a formula (that I cannot add here as an image) but that shows that: From Eq. (11) it is clear that if we want have a low turn radius, that we would expect to need

Large CL (CLmax)
Low altitude (high density)
High load factor n (nmax)
Low wing loading (W/S)

The formula I mentioned indicates that the Radius of Turn is inversely proportional to air density, gravity, and lift coefficient. It is also directly proportional to wing loading and load factor ( n / sqrt (n2 - 1). The latter is a function of bank angle. This formula appears to make more sense than the equation presented before (in the previous answer) where only speed and bank angle appear as variables. If this latter formula was complete, an airplane could turn even when the density is zero (and we know an airplane needs air to fly).

If we imagine an experiment of an airplane flying at high altitude, there will be a point where the air density will be so small that all the lift is needed to keep the airplane flying. In this case, the radius of turn is (almost) infinite: the plane cannot turn or it would lose altitude. If another airplane, very similar but equipped with a larger wing, (thence lower wing loading) was flying, it could generate more lift. So it could bank slightly without losing altitude and therefore it could turn. Here we have a case where the airplane with lower wing loading enjoys a tighter radius of turn.

ALL your three above comments apply ONLY to the MAXIMUM POSSIBLE turn performance (minimum possible turn radius), NOT to the instantaneous turn radius. Instantaneous turn radius, (as described above), is dependant on velocity and radial G, and that is all.  All these other factors only affect the capability of the aircraft to generate lift, which affects its capacity to generate radial G. In other words, all the stuff you're talking about only affects HOW MUCH RADIAL G an aircraft can generate at a particular airspeed, I.e., the MAXIMUM radial G it can produce.

@Aldo, It sounds like you may have mis-stated your question. I think you are actually asking about the  formula for calculating the MINIMUM turn radius an aircraft can generate. If so, then those other factors you mention ARE relevant. But the ACTUAL turn radius at a given airspeed and bank angle (radial G) is NOT, in any way, dependent on them. It's simple. You can calculate the Minimum POSSIBLE turn radius, or you can calculate the ACTUAL radius the aircraft is flying on at any specific instant. For eg., whatever Wing loading, etc., if bank angle is zero, ACTUAL turn radius is infinite!

Charles, first of all, thank you for taking the time to answer my question. It is appreciated. I know you could be doing something else.

Second, since there is a lot of compiled evidence about the difference in turn performance between aircraft (minimum radius of turn without losing altitude) and apparently a general agreement that this performance difference was explained in part by wing loading I wanted to see if somebody had an equation that took every factor into consideration to compute this minimum radius turn. Pilawskii (in his book) mentions that he used a widely employed formula that includes wing loading, Cl, excess power among other factors.

The only formula I could get, included as mentioned before wing loading, load factor, air density, gravity and lift coefficient, so I wanted to see if somebody had a more precise version. I was really surprised that the answers I got were that turn radius (minimum turn radius is a type of turn after all) did not depend on wing loading. The formula where turn radius is calculated based only on velocity and bank angle (which affects radial g) and that was presented as a calculation to predict turn radius appeared to me like a simplification for pilots.

Why? because at the end what makes an aircraft turn is the interaction of two physical things: moving air and a surface. Remove one and there is no force to that allows your vehicle to turn. Imagine that you have a car moving fast and you place a ramp that tilts the automobile. You have the bank angle and the speed but the vehicle will NOT turn because it does not have wings and therefore does not generate a lift force in the desired direction. If you have an aircraft in a bank at speed and suddenly there is no air, the aircraft does not continue the turn. Surely a formula that does not ...

...include the wing and the air appeared to be incomplete (at least for me) because it predicts that the car will turn and the aircraft will also turn in the absence of air. Also, if the mass of the Earth is reduced to the mass of the moon, the radius of turn should change. As a consequence, I have a problem trying to reconcile other formulas that seem to take into consideration factors that in the real world affect the performance of the turn and the other, simple equation where only two factors enter into play.

@Aldo,  You are still missing the point. You are talking about an equation to calculate the MINIMUM POSSIBLE turn radius, but complaining about the existing formula for the ACTUAL turn radius. It's the same as  measuring the HIGHEST that a person can lift his arm, versus the ACTUAL Height of his/her arm at the moment. Or the HIGHEST winds that an aircraft can take off in, versus the ACTUAL winds when it taxies on the runway. Please read my responses more carefully.

... and your analogy with the car on a tilted ramp is totally off base. If the car dos not turn it is only because the turning force is opposed by sufficient sideways friction force from the wheels on the surface. If the car was rolling forward, and the wheels were allowed to free-cast the car would indeed turn and the turn radius would EXACTLY be equal to the Velocity divided by the Radial G. In this analogy the radial G would be equal to the sideways component of the force exerted by the tilted surface on the bottom of the wheels, DEPENDANT on the tilt angle of the surface.

And even if there is no air (in your other analogy), you have to understand that the formula for turn radius is R = V^2/Gr. When you see a formula with bank angle in it instead of radial G (Gr), it a simplification for AIRPLANES that assumes that the radial G is due to the sideways component of the lift generated from the tilted wing.  The actual formula (R=V^2/Gr) is ALWAYS true, for an aircraft, a satellite, the moon, planets, a car on a track, or even for a kid on a merry-go-round. For an aircraft, the Gr is generated by lift because of the bank angle.

I am actually reading your responses carefully. Curiously, I also have the impression that you are not reading mine with sufficient attention but I try to avoid this type of sentimental language because it is not conducive to arriving at proper conclusions (I do not have evidence that you are not reading my comments carefully and neither do you). Second, I am not "complaining" about the equation of ACTUAL TURN RADIUS. (A) Yes, the equation I talked about is to predict the minimum lever turn radius of an aircraft. I am wondering if this equation is complete since at least one author includes

more variables. (B) when I compare the Minimum Turn Radius equation with the Actual Turn radius I noticed two things, first I do not understand the definition of Actual Turn radius, and second, I became intrigued on why using the equation for Minimum Turn Radius it is found that wing loading is a factor but when using the Actual Turn radius it is not. This seems confusing to me, so I went to great lengths to try to explain why it seems odd. In any case, this is my loss and here we are rapidly approaching the point of diminishing returns.

Look at https://www.physicsclassroom.com/class/circles/Lesson-1/Mathematics-of-Circular-Motion. Otherwise, I'm done with you.

The conclusion is that turn performance (defined as the ability of an aircraft to turn in the smallest possible radius without losing altitude) is a function of wing loading, Cl, and bank angle among other factors. For special cases, like coordinated, constant speed-bank angle-altitude turns a general physics equation using speed and bank angle as the only variables can be used to calculate the radius of a turn. However, this simplified formula has limited use to compare the max-turn performance of different aircraft (i.e.an aircraft can turn at a speed where another cannot fly and a maximum

performance turn bleeds speed, thus the equation is not applicable). As Pilawskii correctly explains, excess power is also an essential factor for a minimum radius turn because power needs to be applied to avoid losing altitude.


text:
What do the letters "h" and "i" next to TCAS ND symbols mean?
What is the meaning of the letters h and i seen behind the relative altitude on the TCAS ND indication:

It is from the type-rating material. There isn’t any other information provided than the slide attached above.

It makes no sense. I've never seen n alpha character on that location on a TCAS display. As @757toga commented, the trend arrows are missing. On the map they should be to the right of the symbol. At the bottom, where the h and i are located. Perhaps in generating the presentation material someone got the wrong character set changing the arrows to the letters.

The aircraft type is A320.

They mean the simulator or mockup is buggy and Wingdings 3 was missing / broken / not selected. There should be trend arrows there instead.
First smoking gun: this appears to be a photo of either a simulator or a mockup, as we can see from the Microsoft Windows mouse cursor:

Now, in many avionics software graphics engines - both for flight hardware and simulators (and of course for mockups) - it's common to use special fonts for symbology. In this case I'm guessing the "h" and "i" are meant to be rendered as trend arrows, using such a special font which has those symbols in place of normal letter glyphs.
Second smoking gun: check out Wingdings 3!

Character code 0x68 is "h" in normal fonts. 0x69 is "i" in normal fonts and a down arrow in Wingdings 3.
It should look more like this:

The trend arrows indicate that the target is climbing or descending.

Well done! Bounty on this answer as soon as the system will let me award it (24 hours, I think).

Thanks! I'm a software engineer (and pilot) and worked on professional flight simulators and avionics for 10 years so I had an advantage in spotting it.

Is there a way to crosspost this to StackOverflow? This is a fun confluence of computing and aviation bugs


text:
Do major passenger airline jets have airbrakes/speedbrakes?
Now I've heard that passenger airliners do not generally have airbrakes , and simply use their spoilers as airbrakes since they can perform the same function.
Are there any major jets that use speedbrakes (by major I mean widely used by countries or airlines, and it must be a passenger jet)

Possible Duplicate: For large jets, what is the primary means of slowing down after landing?

You understand that a spoiler is essentially a type of airbrake, right? It's just specifically one that also reduces lift.

@JonStory as you pointed out, they're not the same. Spoilers reduces lift (vertical force), thus if a pilot wishes to maintain vertical speed, AoA has to be increased. When AoA increases, drag increases. Airbrakes increases drag directly by countering the forward momentum of the aircraft. I believe that's what the OP is asking.

Airbrakes also reduce lift (vertical force) - when your aircraft slows down, it reduces lift.... the Spoiler has lift reduction as a primary function with reduction in airspeed secondary, whereas an airbrake is the reverse: the essential point remains that both reduce lift and airspeed.

Yes (for some values of major)

Adrian Pingstone

(c) Harm Rutten

I am not sure if thrust reversers (2nd picture)  would count as conventional airbrakes.

@yankeekilo: redirect your gaze to the rearmost point of the Fokker 70 fuselage.

Hehe, talk of human factors... I was browsing on my smartphone on the go and only registered the reversers :D

@yankeekilo. Actually Concorde used to use its reversers as airbrakes during the descent.

@vectorVictor Very interesting! That would sure merit an answer if not for the demise of the Concorde program.


text:
Why is it said that the Tupolev Tu-144 had superior aerodynamics to the Concorde?
A quote from this article:  "The Concorde had better range, braking and engine control but the Tupolev had vastly superior aerodynamics."
What is the basis, if any, for this statement on the aerodynamics of the two aircraft?

Frankly, that entire article should probably just be discarded. It appears to be written by someone who's briefly scanned the Wikipedia summary box and introduction for the articles of each aircraft...

Let's see if we can find numbers to support this claim:
Tu-144D:
Ogival delta wings without conical camber, cranked delta. A distinctly low set & a much simpler wing, a poor performer at low speed, hence the later addition of canards, with no doubt a big weight penalty.
Concorde production version:
Double delta (ogee/Ogival) shaped wings with conical camber, more curved. Advanced, highly refined wing design.
Comparing the characteristics:
\begin{array}{|l|r|r|c|}
\hline
  \text{} & \text{Tu-144D} & \text{Concorde} & \text{Comments} \\
\hline
   \text{Passenger capacity} & 155 & 120^{[3]} \\
\hline
   \text{Fuel load [kg]} & \text{95,000$^{[1]}$} & \text{95,680}^{[3]} \\
\hline
   \text{Range [km]} & 5330^{[2]} & 7222.8^{[3]} \\
\hline
   \text{Cruise Mach no [-]} & 2.15^{[5]} & 2.04^{[3]} \\
\hline
   \text{Fuel/km [kg/km]} & 17.8 & 13.2 \\
\hline
   \text{Fuel/pass-km [kg/km]} & 0.118 & 0.110 \\
\hline
   \text{L/D (Low speed)} & ? & 3.94^{[3]} \\
\hline
   \text{L/D (Approach)} & ? & 4.35^{[3]} & \text{250 knots, 10,000 ft} \\
\hline
   \text{L/D (Mach 0.94)} & ? & 11.47^{[3]} \\
\hline
   \text{L/D cruise} & 7.1 _{M=2.15}~^{[6]} & 7.14 _{M=2.04}~^{[3]} \\
\hline
   \text{T/W} & 0.44^{[4]} & 0.373^{[3]} \\
\hline
\end{array}
$^{[1]}$ Plane and Pilot - TUPOLEV TU-144 - Source
$^{[2]}$ Tu-144 SST Technical specs - Source
$^{[3]}$ Wikipedia: Concorde - Source
$^{[4]}$ S. Maharajan: Design of Supersonic Transport - Source
$^{[5]}$ NASA Facts: The Tu-144LL: A Supersonic Flying LaboratorySource
$^{[6]}$ TUPOLEV TU-144 - ALL YOU NEED TO KNOWSourceSource
So the specific fuel consumption is very close, with the Tu-144 at 7% higher. And I've used figures for the Tu-144D version which was a big improvement over earlier versions: those could not supercruise at Mach 2, unlike Concorde, which drastically reduced their range.
From Wikipedia:

It is unclear why Tu-144D's maximum achievable altitude would be lower than Concorde's even regular flight altitude, given that Tupolev's data claim better lift-to-drag ratio for the Tu-144 (over 8.0 for Tu-144D vs Concorde's 7.3–7.7 at Mach 2.x) and the thrust of the Tu-144D's RD-36-51 engines is higher than Concorde's Olympus 593 engine.
The lower landing speed compared to Tu-144 is due to Concorde's more refined design of the wing profile that provides higher lift at low speeds without degrading supersonic cruise performance – a feature often mentioned in Western publications on Concorde and acknowledged by Tupolev designers as well.

summary from Tony Buttler, 'Building Concorde':
Early on in the Concorde project, wind tunnel tests found that a wing of the right shape would generate overwing vortices at low speeds and high angles of attack. This meant a big increase in lift, which lowered takeoff and touchdown speeds.
Small strakes on the front fuselage were found to give superior results to canards (the canards interacted with the fin in undesirable ways). A fuel transfer system was used to trim the aircraft instead of relying on aerodynamic trim controls.
The Tu-144 relied on retractable canards for low speeds, while BAC had found a superior solution.
Conclusion:
My impression is that the basic aerodynamics of the Concorde were better. Tupolev tried to compensate by optimizing the surface smoothness at the cost of maintainability (to borrow from the other answer, "high aerodynamic cleanness of TU-144 which was achieved by making the design more complicated and by lowering maintainability of the aircraft"). This meant closing off access hatches, using large complicated skin panels to reduce the number of seams, etc.

I noticed I was looking down and up a lot when reading the characteristics, soI put the numbers in a side by side table. Please roll back if you think it's hurting the quality of the answer.

Unfortunately, there is no hard data available (at least in open literature) to back up these claims.
One for the claim may be due to the fact that the Tu-144 was designed to fly higher than Concorde as its primary flight path was over land rather than water. Because of this, the L/D ratio of the Tu-144 was higher than that of Concorde (7 vs 8 in Tu-144D). Also, the Tu-144 was bigger and was meant to carry more passengers.

Photo Courtesy: globalsecurity.org
The Tupolev site offers an interesting explanation for this claim. Basically, they say that despite the engines and other subsystems being inefficient, the Tu-144 performed similar to the Concorde due to superior aerodynamics of the aircraft. The performance of the Tu-144's RD-36 engines was later brought up to that of Concorde's Olympus 593, at least in fuel consumption.
To quote,

It should be added that the engines were less perfect (by specific parameters the engines of TU-144 a/c were brought close to “Olympus” only in their last modification). All these negative initial data were compensated in the course of development of the project by high aerodynamic cleanness of TU-144 which was achieved by making the design more complicated and by lowering maintainability of the aircraft.

But without any hard data, it is not possible to compare the two aircraft directly.

Also generic explanation in this answer.


text:
Why does a turn and slip indicator matter?
I'm studying the principles of flight, and I'm having some unanswered questions in my mind regarding the turn and slip indicator.
I'm reading that a turn and slip indicator displays the slip or skid of the turn. It probably has something to do with directional stability.
But the questions I'm having are:

Why does it matter if the airplane is in a skidding or slipping turn?
What are the consequences of skidding or slipping turn?
Is it dangerous to be in a skid or slip?
Why would a pilot would like and/or not like to be in a skid or slip?

The turn and slip instruments are combined for convenience and by convention more than anything else, slip is important whether you are in a turn or not. When you slip it means one wing is more directly pointed into the airflow, which increases lift for that wing in comparison to the other. At a high angle of attack, for instance a climb and close to stall speed, a slip can put one wing into a stall, causing the non-stalled wing to flip the airplane. When done deliberately it's an aerobatic maneuver called a snap roll, when it's not deliberate it's a major cause of stall-spin accidents on take-off and approach. So slip is dangerous in some situations.
In level flight slip creates drag, so it's worth keeping the ball in the middle when safety is not a concern as well.
As for deliberate slip as I said before slip can be induced for aerobatic maneuvers like snap rolls and spins, slip can also be used for non-aerobatic maneuvers like side-slips, where you put the airplane out of balance to increase drag, allowing more rapid descents without increasing airspeed.

I would like to add this, it's also one answer for the question: Why does a Turn and slip indicator matter?. My book describes that the turn and slip indicator works as a back-up in case of a malfunctioning artificial horizon / attitude indicator. Is this true in practise?

That's a separate question @Julian, why not ask it?

I did so, please have a look: https://aviation.stackexchange.com/questions/72047/turn-and-slip-indicator-as-a-back-up-for-the-attitude-indicator

The "slip" part of the Turn and Slip Indicator (T/S) measures the lateral (transverse) acceleration of the airplane. When the slip indicator is centered, the pilot and passengers will feel gravity directly in-line with the seats; otherwise, they will feel a lateral sway, which doesn't provide the best ride experience, especially if martini spill is involved. Obviously, this is important whether you are in a turn or simply flying level.
In symmetric flight with multiple engines providing the same thrust, a zero lateral acceleration is very close to having zero aerodynamic sideslip. Achieving flight with minimal sideslip improves the fuel economy since sideslip introduces unnecessary drag for symmetric flight. In practice, however, non-zero sideslip may develop because of minor aerodynamic asymmetry, prop wash, and especially if you have one engine inoperative (even with T/S centered).
Having non-zero slip indication in itself is not dangerous. All Part 23 and 25 aircraft certify by performing steady-heading sideslip up to maximum rudder pedal or the limit of their lateral control. When you decrab in a crosswind, you will intentionally generate sideslip. However, an excessive slip coupled with high angle of attack is dangerous as it can lead to potentially irrecoverable spin-stall.

In a small aircraft, the turn indicator (and the slightly better turn coordinator) is usually electrically powered, while the attitude indicator is vacuum powered. If the attitude indicator fails, either mechanically or do to vacuum fail, the turn indicator can assist with keeping the wings level in instrument conditions.
The slip indicator is merely a ball in a fluid and is not subject to any electric or vacuum failure, and can be a backup for an electronic flight display's slip-skid indicator. In attempting a coordinated turn, the ball can be "stepped on" via the rudder to prevent undesirable yaw. In some aircraft, rudder isn't necessary in a turn or initial bank as yaw is negligible.
Under Instrument Meteorological Conditions (IMC), a skid at or near stall speed indicates a spin (usually undesirable), and the instrument can assist with recovery. A slip at speeds well above stall with rapidly increasing airspeed indicates a spiral. The spiral may be due to misleading attitude indicator, and the indicator and ball can assist in recovery from a spiral.
In a multi-engine aircraft under instrument conditions, yaw (and heading change) is one of the first indications of an engine failure of some sort due to asymmetric thrust, and "stepping on the ball" and raising the wing of the dead engine can restore stable flight.


text:
Can a mistake of a single air-traffic controller cause a collision between planes?
I am watching season 2 episode 13 of Breaking Bad and in the episode it shows that:

an air traffic controller, distraught from his daughter's death,
  causes a mid-air collision, resulting in debris and human remains
  raining down onto the Whites' residence, as well as the rest of
  Albuquerque

It is at the very end of the episode and it looks like he has done nothing malicious just forgot to tell something. To me this sounds really surprising that a single human error can cause such damage.
As a programmer, I know that most of the scenes about computers depicted in the movies are complete nonsense. Is this a similar nonsense about aviation or is this really possible?

Looks like yes --- http://www.youtube.com/watch?v=AZIf_ivdMes

and while on the ground too -- http://www.youtube.com/watch?v=LepaN0QI-Bs

I expect you will like the book Fate is the Hunter

I haven't seen the Breaking Bad episode, but in my experience any aviation related scenes in films and series are usually stretching reality beyond the breaking point.
But it is possible that a single mistake can cause an accident. The probability of that happening is extremely remote, though.
Typically there are a number of safety barriers (also called defensive layers) in the system(1) that prevent any single mistake from developing into a collision. However, every barrier has a small probability of not catching the problem (e.g. due to it not being perfect or being in state of maintenance / failure) so eventually a crash could result from a single mistake.
This is often called the Swiss Cheese Model (originally from Reason, J. (1990) Human Error. Cambridge: University Press, Cambridge.)

When an accident happens, sometimes the phrase "all the holes in the Swiss cheese lined up" is heard, and you now know why.
While the first hole might be the mistake of a controller, the other holes may not be mistakes but are certainly rare circumstances and flaws in the system.  Accident investigation focuses on identifying these contributing factors.
In the Überlingen accident mentioned by Portree Kid, two immediate causes have been identified(2):

The imminent separation infringement was not noticed by ATC in time. The instruction for the TU154M to descend was given at a time when the prescribed separation to the B757-200 could not be ensured any more.
The TU154M crew followed the ATC instruction to descend and continued to do so even after TCAS advised them to climb. This manoeuvre was performed contrary to the generated TCAS RA.

The first cause can be seen as a mistake by the controller, however that mistake could only happen because of a number of contributing factors (systemic causes):

Management and quality assurance of the air navigation service company did not ensure that during the night all open workstations
  were continuously staffed by controllers.
Management and quality assurance of the air navigation service company tolerated for years that during times of low traffic flow at night only one controller worked and the other one retired to rest.

In addition to the above, the Short Term Conflict Alerting (STCA) system did not display conflicts to the air traffic controller because of maintenance (not listed as a cause in the report, but discussed in the text).
The second cause is not a mistake by the crew of the TU154M, but a latent failure in the way ACAS/TCAS II was introduced. Never did anyone give guidance on what to do if the controller and the TCAS gave conflicting instructions.
In the report this is identified as a systemic cause:

The integration of ACAS/TCAS II into the system aviation was insufficient and did not correspond in all points with the system philosophy. The regulations concerning ACAS/TCAS published by ICAO and as a result the regulations of national aviation authorities, operational and procedural instructions of the TCAS manufacturer and the operators were not standardised, incomplete and partially contradictory.

So while the accident was caused by a single mistake from the controller, there were also many flaws in the system, which all together allowed the mistake to develop into an accident.
(1) By system I mean the aviation system as a whole, not only the air traffic control (computer) systems.
(2) Überlingen accident report

Another case of a chain of failures. Clapham Railway disaster http://www.railwaysarchive.co.uk/docsummary.php?docID=36

"Never did anyone give guidance on what to do if the controller and the TCAS gave conflicting instructions. " European (and most anywhere outside Russia) were and are to follow TCAS in such cases, Russian regulations were to leave the decision to the pilot, who typically slavishly (this was shortly after the Soviet era) followed instructions almost without thinking.

I haven't heard of the Swiss cheese model (which describes it quite well), but the accident chain is a very common way to describe the same thing. Break any one link in the chain of events leading up to an accident and you prevent it altogether.

@jwenting Do you have any reference for that? Before 2004, the ICAO requirements did not address the case of TCAS conflicting with ATC. In fact 6 of the 19 recommendations from the investigation are addressing the uniformity and quality of TCAS use worldwide. Also note that a year before the Uberlingen crash, in 2001 a near miss happened in Japan which also involved conflicting instructions between TCAS and ATC. Both aircraft involved were Japanese.

Looking at this as a deliberate act it is really possible, but it would still take the element of chance to make it work. There is technology designed to prevent such an occurrence:

ATC software. The picture that ATC sees is computer generated with lots of extra information, and it looks for potential collisions and alerts for them. I believe the US system sends both audio and visual cues called "deals"
Commercial aircraft above a certain size is almost always equipped with TCAS (Traffic Collision Avoidance System) which will warn pilots about potential collisions and tell them to climb or descend
The human eye: pilots are often looking out and may spot a potential conflict

So a controller wanting to cause a collision could set 2 airplanes without TCAS on a collision course in bad weather or in the dark, then ignore automated conflict messages and cause a very dangerous situation. That doesn't guarantee a collision as there's a lot of airspace up there, so the controller could do all that diligent work and only get a "near miss".
As for accidents yes, controller errors have led to collisions in the past. It's usually a cascade of errors rather than a single error.

At airliner cruise speeds looking out the window doesn't help much. Pilots cannot see up or down, and head-on collisions happen too fast for the crew to react.

@Paul, head-ons are one thing, but collisions in commercial aircraft have been avoided many times by alert pilots when approaching at an angle. Plus, it only take a few feet to avoid a collision. See and avoid is still an important process no matter how much technology there is out there.

@paul that's not really true. Many cockpits have great FOV though seeing directly down or above+behind can be problematic. At altitude visibility can be tremendous and you can sometimes see planes at quite a distance. Of course it helps if the sun is reflecting off the other airplane or that airplane is seeding a contrail. That gives decent time to react and even if you cant see you an listen to the TCAS which gives ample warning of a potential conflict (TA) before giving a direct command needing immediate reaction (RA).

@casey: I agree that the FOV is usually good, but: In PSA 182 the pilots probably (accodring to the documentary; not sure if the WP page mentions it) reduced their field of view by not setting their seats properly and GLO 1907 did not notice each other in head-on collision in good visibility.

If I recall correctly a "deal" is an operational error, the thing alerting everyone to an impending "deal" is called the "snitch" :)

@JanHudec: And, in 1965, see-and-avoid actually caused a MAC (the two aircraft were flying just above a sloped cloud deck, causing it to appear to the pilot of one aircraft that they were on a collision course, even though they actually weren't, causing him to take attempted evasive action which brought the two aircraft onto an actual collision course); fortunately, most of those involved survived the experience.

It not only is possible, but actually occurred over Southern Germany.
 Due to another controller resting one controller was watching two workstations. Just before the crash he became aware of the situation an told one plane to descend. Unfortunately TCAS alerted the other plane to descend, too. The first plane ignored the TCAS instruction to climb and both plane collided killing 71 people. The controller was later murdered by a grieving relative.

The controller was later murdered by a grieving relative. - Wow, that's messed up.

The crash was not caused by him, but rather by a plethora of interacting flaws in procedures and equipment. The one blatant error he made (and that under pressure) was to tell the Russian aircraft that the other plane was coming from the right when in fact it was coming from the left, but at that time it was too late anyway. The main factor in the accident was the Russians' failure to follow international procedures for dealing with TCAS warnings.

@jwenting of course it wasn't the only factor and it was a string of events that led up to it. That is why it is so important to have procedures and to follow them. Like that TCAS overrides ATC. That said I cannot accept him being killed!

@PortreeKid of course not, killing him was cold blooded murder, honouring him for that murder just shows how criminal that entire country is. I think one of the results of the investigation was a change in Russian procedures to align their TCAS handling with the rest of the world. Just like a result of Tenerife was to introduce common phraseology and procedures worldwide for runway entry and takeoff clearance.

@jwenting: The factor was that the procedures were not established at the time yet. The investigation report does not list this as fault of the Tu154 crew, but as lack of such rule. Of course introducing that rule is in recommendation in the report and it was introduced and in any new accident it would be fault of the pilot if they didn't follow TCAS.

@JanHudec the crew of the Tu acted according to their training so can't be held at fault even though their action flew their aircraft into the DHL one. Yes, their procedure was wrong, but it was wrong due to no fault of their own (though maybe they could/should have known that outside Russia it's the norm to obey TCAS in case it didn't match ATC instructions, but that's speculation).

This one goes back a ways, but yes. (early 70s) Center split up a flight of 4 F-4s into 2 flights of 2 over Florida, stacked them over a navaid in the holding pattern. Controller confused the call signs and decended the upper flight of 2 through the 2 holding a thousand feet below. One F-4 lost the vertical stabilizer, both pilots landed safely after a nylon let down. The other F-4 in the mid air lost 6 feet of wing and landed safely engaging the approach end BAK-9 arresting cable.

Sure.  I have been given an incorrect heading to fly that could have been tragic (fly into mountain or another aircraft).  Just like pilots, controllers can lose Situation Awareness.  Especially given their schedules that have them working crazy shift with minimal down/sleep time.


text:
Was there a design flaw in the fuel tanks of the MiG-15/17/19/21?
Wikipedia article on MiG-15:

Another major design deficit was that the tank could develop under-pressure if more than half the fuel had been used, which could easily lead to tank implosions, destroying the aircraft.

MiG-17:

Like its forebear, the MiG-17 inherited the major design deficit which could cause the fuel tanks develop to under-pressure if more than half the fuel had been used, which could easily lead to tank implosions, crushing the main fuselage of the aircraft mid-fight, with almost always fatal results.

MiG-19:

Initial enthusiasm for the aircraft was dampened by several problems, the most alarming inherited from MiG-15/MiG-17 was the danger of mid-air tank implosions when more than half of the fuel had been used - the leaking fuel of the crushed fuselage fuel tanks located between the engines would then ignite, leading to a fatal explosion;

MiG-21:

Additionally when more than half the fuel was used up, violent maneuveurs prevented fuel from flowing into the engine, thereby causing it to shutdown midflight.3 and risking tank implosions (a problem inherited from MiG-15/MiG-17 and MiG-19).

None of these entries has a citation.
I would like to know if these aircraft really did have a design flaw in the fuel tanks, and if so, what was it? What was wrong with the tank that made it susceptible to implosions once more than 50% of fuel was gone?

can you check the history to see if it was edited in by the same guy? Sounds like something that needs a [citation needed] tag on all articles and perhaps a discussion in the talk pages

@ratchetfreak I suppose I could hunt through the histories of all 4 pages, but I'm not really interested in who wrote them or if it was the same guy. As far as I'm concerned, they need citations no matter who wrote them.

I found a reference to exploding fuel tanks on Mig-21's caused by overheating engines.
 "it had an unfortunate tendency to explode in mid-air. The problem was finally traced to a fuel tank placed under the engines that could cook up when the engines overheated."

Like its forebear, the MiG-17 inherited the major design deficit which
  could cause the fuel tanks develop to under-pressure if more than half
  the fuel had been used, which could easily lead to tank implosions,
  crushing the main fuselage of the aircraft mid-fight, with almost
  always fatal results.

Tank implosion implies negative tank pressure. Under pressure or negative pressure is caused by improper fuel tank venting.
High pressure pumps sucking fuel out of an improperly vented tank can quickly develop negative pressure which can cause a tank collapse. If the tank was integral to the fuselage it could also cause an airframe failure. A collapsed tank could also leak fuel which could lead to catastrophic failure if the fuel found an ignition source.

Now the MiG-21 article has this book listed as source:
Dunnigan, James F. How to Make War, A Comprehensive Guide to Modern Warfare in the 21st Century, Fourth Edition. Harper Collins Publishers Inc. 2003.

@Therac He's saying yes, the MiG-15, -17, -19, and -21 have the flaw. The blockquote is in my OP is under MiG-21. He's saying this blockquote is now sourced in the MiG-21 wiki article. I checked it and indeed it is. I kinda want to edit his answer anyway and say this explicitly, but maybe it's better to give people a chance and learn how to edit. I upvoted anyway because he gives a source.

I see! It wasn't obvious at first what exactly the answer meant.


text:
Can the MH-60 helicopter fly without one or all of the stabilators?
The MH-60R helicopter has a horizontal Stabilator that consists of three subassemblies, RH Stabilator, LH Stabilator, and Center Stabilator.
Can the aircraft fly without the RH Stabilator?
Can the aircraft fly without the entire Stabilator?

Tail rotor still intact?

can you be more specific?  by "without" do you mean broken? not installed? not locked into position?  and by "fly" do you mean all regimes of flight from takeoff to landing? or do you mean can the copter continue to be flown (even to a landing) if the stabilator is damaged in flight?

Assuming there is no STC ("Supplemental Type Certificate") that addresses this as an existing modification, the answer, in terms of acceptable aircraft airworthiness, is a hard no.
"Can the aircraft fly" is a tricky question in some ways, but dead-easy in others.  And remember: a helicopter is a thousand moving parts spinning wildly around an oil leak.
If you're looking at it from a flight physics and dynamics perspective, as a result of a mishap/damage in flight, (assuming no other problems like a damaged rotor or loss of hydraulic fluid,) and whether you can control the aircraft safely to the ground, a skilled pilot rated for this aircraft "type" would probably be able to wrestle the aircraft to a safe landing.  The main rotor does the work of holding the helo aloft, and the tail rotor holds the attitude steady against the torque of the main rotor.  The stabilator (stabilizer-elevator combined surface) does not contribute to total lift in hover, although it may affect lift in forward flight.
However, any mission the helo had been in the midst of should be immediately abandoned for the sake of making the landing.  Even if you were rescuing someone from drowning in the ocean, you're more likely to just add the flight crew to the tally of the dead if you persist in trying to make the bird work while in an adverse configuration.
This does not answer what an actual pilot of that type - which I am not - would experience in flight.  But the stabilator is considered a flight control surface, and you never take to the air without your flight controls 100% operational.
But ending up missing part of your flight control surfaces during flight would be considered an emergency condition.  If instead the aircraft is already on the ground, in need of maintenance, and one is considering a "ferry flight" to somewhere else to better service it, there are many things that an inspector could let slide for just a ferry flight, but this is not one of them.
Such a stabilator contributes to the control, stability, and balance of the aircraft, and without the entire stabilator, the aircraft is not in flight configuration, and therefore, it's not "airworthy".
Definition of Airworthiness:

The aircraft conforms to its type design and is in a condition for safe operation.

Source: 14 CFR Part 3.5(a)
Even if a cogent argument is somehow made that the helicopter is "in a condition for safe operation" while lacking all or part of its stabilator, it still utterly fails the first half, being: "conforms to its type design".
A note regarding balance:
Many mid-sized helicopters have an acceptable CG (Center of Gravity) range that's about the size and shape of a large mobile phone (as I write this in early 2016).  This is a larger craft, flown by the military, so its CG envelope may be the size of a small to medium tablet device.  Loading various people near the CG location doesn't shift the CG very much, which is why the main passenger/cargo area is always under the main rotor.  But that stabilator flight control surface could weigh 20, 50, 100 pounds or more, and removing that from the end of a long lever arm at the end of the tail boom will cause a noticeable shift in CG location, possibly putting it out of range for the aircraft.  This is also a situation that a skilled pilot in an emergency may be able to compensate for if action is taken immediately, but is a condition you never enter into knowingly. That way lies only heartache and doom.
I am an FAA certified A&P (Airframe and Powerplant) mechanic, and during my training school, the third-ranking member in the entire FAA (in 2007) came and gave a talk on a few different topics, but primarily on the concept of airworthiness.  It comes down to this:  Are you SURE it will fly safely?  How sure?  Would you put your kids in that aircraft and let it fly?  As aircraft mechanics, we need to be that sure that the aircraft is safe to fly, or, it simply isn't.

An aircraft is fully airworthy, or it's not airworthy.

The Sikorsky SH-60 Seahawk, without a complete stabilator, is not airworthy.

FWIW, the Navy went all goofy on nomenclature and changed  all of the SH's to MH's.  (Having flown the SH-60B I'll admit to having rolled my eyes a bit on that when it happened ...)  Might want to correct your last sentence, as the question is about the MH-60R (which back when it was originally envisioned as the next version was SH-60R ... but that's a while back ...)

This has happened.  The aircraft was landed (as soon as possible) for a good reason.
Both of these cases address partial failure.  Complete loss of stab I have no knowledge of.
The RH failure happened once that I know of at low speed (stab full down) on an SH-60B (1980's).  It created a right stick / cross control situation (left roll) that the pilots were able to overcome.  The aircraft was landed and was not flown until repaired.  (Source: a guy I know who was part of the flight test crew; he was in the back of that bird monitoring test equipment).   (Note: SH-60B and MH-60R have the same stab).
The same source is familiar with a Black Hawk (UH-60) that had a structural failure of the stab in flight.  As the in-flight loads tend toward the RH side (due to torque, per his description) and the LH side was what failed, they returned to base and did a run on landing.  That aircraft also was not flown again until repaired.
So yes, under those two discrete failure modes the helicopter "was able to fly" but
WARNING
the aircraft will handle differently.  The change in controllability can create significant hazards to safe flight.
Therefore, the answer regarding that "with a failed stabilator it is not airworthy" is spot on.  The helicopter is not airworthy, it should not be flying.
Get it on the ground.
Fix it before you fly it again.
The stabilator is a flight control surface.

Get it on the ground, absolutely. Good advice. An MH-60 with a (partially) failed stabilator: is it airworthy? No. Should it fly? No. Can it fly? Yes, probably. Would anyone want to try it out? I doubt it.

All modern day helicopters have horizontal stabilisers to provide static stability. The main rotor and the fuselage both have an inherent negative stability derivative in pitch.
The MH-60 stabilator is a stabiliser with variable pitch, set to large pitch angles in hovering flight to help reduce download effects, and lower pitch angles during forward flight to provide stability.
There have been helicopters without a horizontal stabiliser, the ones designed before 1960 generally did not have one. These helicopters can obviously fly, but require constant adjustment of the cyclic longitudinal stick in forward flight, like a "normal" helicopter in the hover. The stabilator provides an order of magnitude improvement in the flying qualities in forward flight.
The MH-60 could probably be flown without the stabilator, however the pilot will need to re-learn flying the aircraft in a hurry. It will be very temperamental.
Sources:

Helicopter Performance, Stability and Control by Raymond Prouty
Principles of Helicopter Aerodynamics by J. Gordon Leishman.


text:
At what altitude does aerodynamic forces become negligible?
If I sail on a boat from North to South the boat will be dragged by the oceans which it floats on, and the oceans are dragged by the Earth. Now when you go higher let say 10 km an airplane will also be dragged by the atmosphere, so the plane would "feel" an aerodynamic force against the atmosphere (rotating around Earth) if it is stationary.
But when you get higher and higher the atmosphere will get less dense and the force of the atmosphere will drop, till you probably reached satellite level where there is almost no drag of the rotation of the Earth at all.
What is the altitude an airplane can no longer "feel" aerodynamic forces because the atmosphere is so thin?

This question is not clear as you state yourself that even satellites feel atmospheric drag (in fact the ISS has to readjust its orbit every now and then): http://space.stackexchange.com/questions/9087/how-often-does-iss-require-re-boosting-to-higher-orbit

The question is when an airplane starts the undergo a difference compared to a boat.

What does a plane have to do with a boat?

Well a boat goes with the flow of water (his 'medium') and an airplane is considered to go with the flow of the air. Both the water and the air are dragged by the rotation of the Earth without any difference. But the question is now at what height the air get so empty that the force of the air is not equal to the rotation of the Earth.

@Marijn: "The force of the air is not equal to the rotation". What is that supposed to mean? You compare a rotation to a force? Anyway, all the air rotates, there is no "rest" air (there is no "rest" at all)

But if there are satellites which are more or less undependent from the rotation of the Earth but still undergoes a friction due to litttle air there must be a level where this all starts...

It's like asking at which time of the night obscurity starts in the middle of the Sahara. There is no single answer, obscurity needs to be defined prior to answering.

It's difficult to understand the question, but if I have, it's based on a complete misunderstanding of the physics involved.  Drag doesn't happen because of the rotation of the Earth, it happens because of the relative motion of the airplane and air masses.  So your plane can be sitting on a parking apron, and (if not properly tied down) experience enough drag in high winds to flip it over.  E.g.  http://www.alamy.com/stock-photo-two-aircraft-were-blown-away-by-storm-emma-at-salzburgs-airport-austria-98044187.html

Not sure this is helpful, but the SR-71 has sustained flight records around 85,000 feet (where something like 500,000 feet is low earth orbit). I'm guessing that somewhere between 85K and 100K feet the air becomes so thin that wings don't meaningfully provide any lift. There probably isn't a hard limit, as you can have different wing designs and different ways of propelling the aircraft. See also -- http://aviation.stackexchange.com/questions/9188/what-determines-the-maximum-altitude-a-plane-can-reach

OK, is see this an old question.  But I have to say, it is not a good question.  Re "if it is stationary."-- why would an aircraft be stationary within a moving atmosphere?  Are you asking about a tethered aircraft?  This question seems to fundamentally misunderstand the basic Newtonian principles of motion.  It would be improved by deleting the whole first paragraph.

Basically, you are asking the altitude at which the atmospheric forces on the aircraft (lift and drag) become negligible i.e. the point where the density of the atmosphere is so low that the impact of air molecules on the aircraft (or spacecraft as it may be) has little effect.
Generally speaking, as the altitude increases, the density decreases, but there is no fixed altitude above which it disappears- rather it gradually disappears into irrelevance. Even spacecraft hundreds of kilometers above earth's surface in low earth orbits (LEO) have to fire their boosters periodically to overcome atmospheric drag.
The image below shows the height of International Space Station (ISS) over time, which shows the loss of height due to (among other things) atmospheric drag.

This plot shows the orbital height of the ISS over the last year. Clearly visible are the re-boosts which suddenly increase the height, and the gradual decay in between. The height is averaged over one orbit, and the gradual decrease is caused by atmospheric drag; Image from heavens-above.com
Even at that altitude, the drag can be significant for the fuel available and as NASA points out,

For most of the last decade, ... the International Space Station they were circling the globe at an altitude of approximately 220 statute miles, or about 350 kilometers.
... European Space Agency’s resupply ship Johannes Kepler... will bring the fuel needed to boost the station to its normal planned altitude of 248 miles, or 400 kilometers.
“As solar activity rises, the atmospheric density in our altitude range increases causing increased drag on the vehicle. This in turn causes us to have to raise the orbit more often.”
Even though the space station orbits in what most people on Earth would consider to be the “vacuum of space,” there still are enough atmospheric molecules ... that the cumulative effect of these tiny particles contacting its surfaces reduces its speed and causes a minute but continuous lowering of its altitude, or height above the Earth.

The question now becomes different- at what altitude does effects other than drag become much more important. Though we cant set a precise value as above, it is possible to set a theoretical altitude above which the drag can be neglected (not entirely) and other things become more important. This is called the Kármán line, which Von Kármán explains as:

... flew 2000 miles per hour (3,200 km/h) at 126,000 feet (38,500 m), or 24 miles up. At this altitude and speed aerodynamic lift still carries 98 per cent of the weight of the plane, and only two per cent is carried by centrifugal force, or Kepler Force, as the space scientists call it. But at 300,000 feet (91,440 m) or 57 miles up this relationship is reversed because there is no longer any air to contribute lift. Only centrifugal force prevails. This is certainly a physical boundary ...

(Quote from Von Kármán's autobiography, from Wikipedia)
As the drag force is similar to lift (both are proportional to the square of speed and directly proportional to density), it only makes sense to consider that the drag loses its effect on the air/space craft, at least for ease of definition.
From Fédération Aéronautique Internationale:

Astronautics needed the lack of atmosphere to be viable; Aeronautics needed the presence of atmosphere. And atmosphere existed near the Earth’s ground, but did not exist far above the ground. In Astronautics, speeds impossible to maintain in atmospheric drag could be kept for very long periods without power applied to the vehicle.
In Aeronautics, level flying higher and higher meant to deal with less and less dense atmosphere, thus to the need of greater and greater speeds to have the flying machine controllable by aerodynamic forces. A speed so big in fact, that, above a certain altitude, could be close or even bigger than the circular orbital speed at that altitude ...
A lot of calculations were made, and finally it was reached the conclusion, accepted by all scientist involved, that around an altitude of 100 Km. the boundary could be set.
The 100-Km altitude, ever since named the “Karman Line”, came thus into existence as the boundary separating Aeronautics and Astronautics.

Basically, for the purposes of your question, we can take the Kármán line as the point where the atmospheric drag loses its effect on the aircraft, which becomes a spacecraft.

There are two intermingled questions here: effects of atmospheric drag, and effects of the earth's rotation.
The effect of atmospheric drag decrease with altitude, but they do not have a fixed "start" nor "end" point.
The effects of the earth's rotation on aircraft in flight are too slight to be noticed and are of no practical concerns, as flight itself happens in an earth-centric frame of reference. Theoretical interest, maybe, but no practical use, in my experience.

At least the starting point of decreasing drag is well defined. It starts at 0 m altitude.

@bogl  Slightly lower than that, if you start from Death Valley, CA or the Dead Sea!

I had altitude over ground level in mind. With reference to sea level, you are totally right.

"The effects of the earth's rotation on aircraft in flight are too slight". Which effects? (The only ones I imagine are affecting wind direction, and they are quite strong.)

The question is about aerodynamic forces

@user40476 As originally asked, both elements were part of the question. The rotational effects fell out with the various edits.

May be aSe needs to be improved so that an edited question (shows under the previous one) , idem for the  answers

It's the altitude an airplane will never reach
An airplane flies because of "drag" (because of induced/reduced air-pressure). If an airplane wants to fly, it needs a certain amount of air (airplane). As long as there is air, there is drag.
Equivalent to a boat: on which altitude has a boat to be to not be dragged by the water: Above the water. But a vehicle above the water is merely a boat anymore...
For a more theoretical answer (neglecting that an airplane by "definition" is not able to reach this altitude), find it yourself with this question: For which r (radius; distance to earth) is 1/r^2 == 0. You may see the problem? The answer is "never".

Perhaps the question was not emphasized enough on the word starts, so this could even be at an heigth of 10 meter above sea level. I know that an airplane flies because there is air and when there is no air there is no drag, but that is not the question. The question is when the air starts losing a bit of drag compared to the rotation of the Earth. But perhaps I missed or mixed something....?